Liquid-gas cryogenic energy storage units operating at constant temperature

The cryogenic energy storage unit described in this article is a device that is able to absorb heat at constant temperature and that provides some significant advantages over the cryogenic storage units working at the triple point. It consists in a low temperature cell coupled to a relatively large expansion volume at room temperature. The heat is absorbed thanks to liquid evaporation, rendering this device able to operate at any temperature along the saturation curve, which is an advantage in respect to the triple point devices. Moreover the large latent heat of evaporation allows a compact low temperature cell. During the period where the heat is absorbed, the constant temperature is obtained by an ON-OFF pressure control using the expansion volume as a vacuum ballast. This concept was tested using nitrogen as working fluid, a 38 cm(3) low temperature cell and expansion volumes of 5.7 L and 24 L Because some possible applications of such devices need their integration in the thermal bus of satellites, the low temperature cell was turned gravity independent by filling it with ceramic foam to retain the liquid by capillary effect. This experimental setup was tested at various temperatures in the 64-81 K range, and as much as 3.5 kJ stored with a thermal stability of around 0.1 K was obtained. A disadvantage of such an Energy Storage Unit is the relatively large room temperature volume: its size is discussed as a function of both the controlled temperature and the working gas. (C) 2015 Elsevier Ltd. All rights reserved.